High performance MTJ elements for STT-RAM and method for making the same

ABSTRACT

An STT-MTJ MRAM cell utilizes transfer of spin angular momentum as a mechanism for changing the magnetic moment direction of a free layer. The cell includes an IrMn pinning layer, a SyAP pinned layer, a naturally oxidized, crystalline MgO tunneling barrier layer that is formed on an Ar-ion plasma smoothed surface of the pinned layer and, in one embodiment, a composite tri-layer free layer that comprises an amorphous layer of Co 60 Fe 20 B 20  of approximately 20 angstroms thickness formed between two crystalline layers of Fe of 3 and 6 angstroms thickness respectively. The free layer is characterized by a low Gilbert damping factor and by very strong polarizing action on conduction electrons. The resulting cell has a low critical current, a high dR/R and a plurality of such cells will exhibit a low variation of both resistance and pinned layer magnetization angular dispersion.

This is a Divisional Application of U.S. patent application Ser. No.11/880,583, filed on Jul. 23, 2007, which is herein incorporated byreference in its entirety and assigned to a common assignee.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a current perpendicular to plane(CPP) magnetic random access memory (CPP-MRAM) cell formed using amagnetic tunneling junction (MTJ) as the basic memory element, wherein aspin torque transfer (STT) effect is used to change the magnetizationdirection of the MTJ ferromagnetic free layer.

2. Description of the Related Art

The conventional magnetic tunneling junction (MTJ) device is a form ofultra-high magnetoresistive device in which the relative orientation ofthe magnetic moments of parallel, vertically separated, upper and lowermagnetized layers controls the flow of spin-polarized electronstunneling through a very thin dielectric layer (the tunneling barrierlayer) formed between those layers. When injected electrons pass throughthe upper layer they are spin polarized by interaction with the magneticmoment of that layer. The majority of the electrons emerge polarized inthe direction of the magnetic moment of the upper layer, the minoritybeing polarized opposite to that direction. The probability of such apolarized electron then tunneling through the intervening tunnelingbarrier layer into the lower layer then depends on the availability ofstates within the lower layer that the tunneling electron can occupy.This number, in turn, depends on the magnetization direction of thelower electrode. The tunneling probability is thereby spin dependent andthe magnitude of the current (tunneling probability times number ofelectrons impinging on the barrier layer) depends upon the relativeorientation of the magnetizations of magnetic layers above and below thebarrier layer. The MTJ device can therefore be viewed as a kind ofmulti-state resistor, since different relative orientations (e.g.parallel and antiparallel) of the magnetic moments will change themagnitude of a current passing through the device. In a common type ofdevice configuration (spin filter), one of the magnetic layers has itsmagnetic moment fixed in direction (pinned) by exchange coupling to anantiferromagnetic (AFM) layer, while the other magnetic layer has itsmagnetic moment free to move (the free layer). The magnetic moment ofthe free layer is then made to switch its direction from being parallelto that of the pinned layer, whereupon the tunneling current is large,to being antiparallel to the pinned layer, whereupon the tunnelingcurrent is small. Thus, the device is effectively a two-state resistor.The switching of the free layer moment direction (writing) isaccomplished by external magnetic fields that are the result of currentspassing through conducting lines adjacent to the cell. Once the cell hasbeen written upon, the circuitry must be able to detect whether the cellis in its high or low resistance state, which is called the “read”process. This process must both measure the resistance of thewritten-upon cell and then compare that resistance to that of areference cell in a fixed resistance state, to determine if thewritten-upon cell is in its high or low state. Needless to say, thisprocess also introduces some statistical difficulties associated withthe variation of resistances of the cells.

FIG. 1 is a highly schematic drawing showing an overhead view of aconventional MRAM cell comprising an MTJ cell element (1000) positionedbetween (or at the intersection of) vertically separated orthogonal word(200) and bit (100) lines. The cell element (1000) is drawn with aslightly elliptical horizontal cross-section because such an anisotropicshape (“shape anisotropy”) produces a corresponding magnetic anisotropywithin the free layer that assists its magnetic moment in retaining athermally stable fixed position after switching fields have been turnedoff. The direction along the free layer in which it is energeticallyfavorable for the moment to remain and from which it should be difficultto switch the magnetic moment unintentionally (as with thermal effects),the longer direction in this case, is called the “easy axis” of thelayer. The axis perpendicular to the easy axis is called the “hardaxis.” The fields produced by currents in each of the two lines arebetween about 30 to 60 Oersteds in magnitude. According to the diagram,the word line field will be along the easy axis of the cell element, thebit line field will be along the easy axis.

The use of magnetic fields externally generated by current carryinglines (as in FIG. 1) to switch the magnetic moment directions becomesproblematic as the size of the MRAM cells decreases and, along withtheir decrease, so must the width of the current carrying lines. Thesmaller width lines require greater currents to produce the necessaryswitching fields, greatly increasing power consumption.

For this reason, a new type of magnetic device, called a spin transferdevice, described by Slonczewski, (U.S. Pat. No. 5,695,164) andCovington (U.S. Pat. No. 7,006,375), has been developed, that seems toeliminate some of the problems associated with the excessive powerconsumption necessitated by external switching fields. The spin transferdevice shares some of the operational features of the conventional MTJcell (particularly the read process) described above, except that theswitching of the free layer magnetic moment (the write process) isproduced by passage of the spin polarized current itself. In thisdevice, unpolarized conduction electrons passing through a firstmagnetic layer having its magnetic moment oriented in a given direction(such as the pinned layer) are preferentially polarized by their passagethrough that layer by a quantum mechanical exchange interaction with thepolarized bound electrons in the layer. Such a polarization can occur toconduction electrons that reflect from the surface of the magnetizedlayer as well as to those that pass through it. The efficiency of such apolarization process depends upon the crystalline structure of thelayer. When such a stream of polarized conduction electrons subsequentlypass through a second magnetic layer (such as the free layer) whosepolarization direction is not fixed in space, the polarized conductionelectrons exert a torque on the bound electrons in the magnetic layerswhich, if sufficient, can reverse the polarization of the boundelectrons and, thereby, reverse the magnetic moment of the magneticlayer. The physical explanation of such a torque-induced reversal iscomplicated and depends upon induction of spin precession and certainmagnetic damping effects (Gilbert damping) within the magnetic layer(see Slonczewski, below). If the magnetic moment of the layer isdirected along its easy magnetic axis, the required torque is minimizedand the moment reversal occurs most easily. The use of a currentinternal to the cell to cause the magnetic moment reversal requires muchsmaller currents than those required to produce an external magneticfield from adjacent current carrying lines to produce the momentswitching. Much recent experimental data confirm magnetic momenttransfer as a source of magnetic excitation and, subsequently, magneticmoment switching. These experiments confirm earlier theoreticalpredictions (J. C. Slonczewski, J. Magn. Mater. 159 (1996) LI, and J. Z.Sun, Phys. Rev. B., Vol. 62 (2000) 570). These latter papers show thatthe net torque, Γ, on the magnetization of a free magnetic layerproduced by spin-transfer from a spin-polarized DC current isproportional to:

Γ=sn_(m)x (n_(s)x n_(m)),  (1)

Where s is the spin-angular momentum deposition rate, n_(s) is a unitvector whose direction is that of the initial spin direction of thecurrent and n_(m) is a unit vector whose direction is that of the freelayer magnetization and x symbolizes a vector cross product. Accordingequation (1), the torque is maximum when n_(s) is orthogonal to n_(m).

Referring to FIG. 2, there is shown a schematic illustration of anexemplary prior art MTJ cell element (such as that in FIG. 1) beingcontacted from above by a bit line (100) and from below by a bottomelectrode (300). The bottom electrode is in electrical contact, througha conducting via (80), with a CMOS transistor (500) that providescurrent to the MTJ element when the element is selected in a read orwrite operation.

Moving vertically upward from bottom electrode to bit line this priorart storage device consists of an underlayer (1), which could be a seedlayer or buffer layer, an antiferromagnetic pinning layer (2), asynthetic antiferromagnetic (SyAF) pinned reference layer (345),consisting of a first ferromagnetic layer (3), a non-magnetic spacerlayer (4) and a second ferromagnetic layer (5), a non-conductingtunneling barrier layer (6), a ferromagnetic free layer (7) and anon-magnetic capping layer (8). Arrows, (20) and (30), indicate theantiparallel magnetization directions of the two ferromagnetic layers(3) and (5) of the SyAF pinned layer (345). The double-headed arrow (40)in free layer (7) indicates that this layer is free to have its magneticmoment directed in either of two directions.

Referring again to FIG. 2 it is noted that when a certain criticalcurrent (arrow (50) is directed from bottom to top (layer (1) to layer(8)), the free layer magnetization (40) would be switched to be oppositeto the direction of the reference layer's magnetization (30) by thespin-transfer torque. This puts the MTJ cell into its high resistancestate.

Conversely, if the current is directed from top to bottom (60), the freelayer magnetization (40) would be switched, by torque transfer ofangular momentum, to the same direction as that of the pinned referencelayer's direction (30), since the conduction electrons have passedthrough that layer before entering the free layer. The MTJ element isthen in its low resistance state.

Referring again to FIG. 2, this entire configuration represents aschematic diagram of a single spin-RAM memory cell that utilizes thespin transfer effect (denoted hereinafter as an STT-RAM) for switchingan MTJ type element. In this paper, we will use the term “element” todescribe the basic MTJ structure comprising a tunneling barrier layersandwiched between ferromagnetic fixed and free layers. We shall use theterm “memory cell” to denote the combination of the MTJ elementincorporated within the circuitry shown that permits the element to bewritten on and read from. Such circuitry includes intersecting currentcarrying lines that allow a particular element to be accessed and also aCMOS transistor that allows a current to be injected into the element.The word line provides the bit selection (i.e., selects the particularcell which will be switched by means of a current passing through itbetween the bit line and the source line) and the transistor providesthe current necessary for switching the MTJ free layer of the selectedcell. Although it is not shown in this simplified figure, the cell isread by applying a bias voltage between the bit line and source line,thereby measuring its resistance and comparing that resistance with astandard cell in the circuit (not shown).

The critical current for spin transfer switching, I_(c), is is generallya few milliamperes for an 180 nm sub-micron MTJ cell (of cross-sectionalarea A approximately A=200 nm×400 nm). The corresponding criticalcurrent density, J_(c), which is I_(c)/A, is on the order of several 10⁷Amperes/cm². This high current density, which is required to induce thespin transfer effect, could destroy the insulating tunneling barrier inthe MTJ cell, such as a layer of AlOx, MgO, etc.

The difference between an STT-RAM and a conventional MRAM is only in thewrite operation mechanism; the read operation is the same for both typesof cell. In order for the spin transfer magnetization mechanismswitching to be viable in the 90 nm MTJ cell structure and smaller, thecritical current density must be lower than 10⁶ A/cm² if it is to bedriven by a CMOS transistor that can typically deliver 100μA per 100 nmof gate width. For STT-RAM applications, the ultra-small MTJ cells mustexhibit a high tunnel magnetoresistance ratio, TMR=dR/R, much higherthan the conventional MRAM-MTJ that uses AlOx as a tunneling barrierlayer and has a NiFe free layer. Such MRAM-MTJ cells have a dR/R˜40%. Ithas recently been demonstrated (S. C. Oh et al., “Excellent scalabilityand switching characteristics in Spin-transfer torque MRAM” IEDM2006288.1 and “Magnetic and electrical properties of magnetic tunneljunction with radical oxidized MgO barriers,” IEEE Trans. Magn. P 2642(2006)) that a highly oriented (001) CoFe(B)/MgO/CoFe(B) MTJ cell iscapable of delivering dR/R>200%. Furthermore, in order to have asatisfactory “read margin”, TMR/(R_(p)covariance), where R_(p) is theMTJ resistance for parallel alignment of the free and pinned layers, ofat least 15 and preferably >20 is required. It is therefore essential tofind a method of fabricating the CoFe(B)/MgO/CoFe(B) MTJ cell with agood read margin for read operation. Note, “R_(p)covariance” indicatesthe statistical spread of R_(p) values.

In MRAM MTJ technology, R_(p) is as defined above and R_(ap) is the MTJresistance when the free and pinned layers have their magnetizationsaligned in an antiparallel configuration. Uniformity of the TMR ratioand the absolute resistance of the cell are critical to the success ofMRAM architecture since the absolute value of the MTJ resistance iscompared to the resistance of a reference cell during the readoperation. If the active device resistances in a block of memory show ahigh variation in resistance (i.e. high R_(p) covariance, or R_(ap)covariance), a signal error can occur when they are compared with thereference cell. In order to have a good read marginTMR/(R_(p)covariance), should have a minimum value of 12 and mostpreferably be >20.

To apply spin transfer switching to the STT-RAM, we have to decreaseI_(c) by more than an order of magnitude. The intrinsic critical currentdensity, J_(c), is given by Slonczewski (J. C. Slonczewski, J. Magn.Mater. 159 (1996) LI,) as:

J_(c)=2eαM_(s)t_(F)(H_(a)+H_(k)2πM_(s))/hη  (1)

where e is the electron charge, α is the Gilbert damping constant, t_(F)is the free layer thickness, h is the reduced Planck's constant, η isthe spin-transfer efficiency (related to the tunneling spin polarizationfactor of the incident spin-polarized current), H_(a) is the externalapplied field, H_(k) is the uniaxial anisotropy field and 2πM_(s) is thedemagnetization field of the free layer. Normally the demagnetizationfield is much larger than the two other magnetic fields, so equation (1)can be rewritten:

I_(c)˜αM_(s)V/hη,  (2)

where V is the magnetic volume, V=M_(s)t_(F)A, which is related to thethermal stability function term, K_(u)V/k_(b)T, which governs thestability of the magnetization relative to thermally-inducedfluctuations.

M. Hosami (“A novel nonvolatile memory with spin torque transfermagnetization switching: Spin RAM” 2005 IEDM, paper 19-1), discusses aSpin-RAM 4 Kbit array which is fabricated using a stack of the followingform: CoFeB/RF-sputtered MgO/CoFeB with a MnPt pinning layer. This MTJstack is processed using 350° C., 10KOe annealing. The cell size is a100 nm×150 nm oval. Patterning of such sub 100 nm oval MTJ elements isdone using e-beam lithography. The tunnel barrier layer is (001)crystallized MgO whose thickness is less than 10 angstroms for thedesired RA of about 20 Ω-μm². Intrinsic dR/R of the MTJ is 160%,although under operational conditions (0.1 V bias, for readdetermination) it is about 100%. Using a current pulse width of 10 ns,the critical current density is about 2.5×10⁶ A/cm². The amounts to acritical current of 375 μA. The distribution of write voltages for thearray, for the high resistance state to the low resistance state hasshown a good write margin. Resistance distributions for the high and lowresistance states has a sigma (R_(p) covariance) around 4%. Thus, underoperational conditions, (TMR/R_(p) covariance) is 25. This is equivalentto the conventional 4-Mbit CoFeB/AlOx/NiFe MTJ-MRAM in which dR/R (0.3Vbiased) typically is about 20-25%. For a R_(p) covariance=1%, TMR/(R_(p)covariance) is >20.

S. C. Oh et al., cited above, describes an STT-RAM utilizing spin torquetransfer where a CoFeB/RF-sputtered MgO/CoFeB was processed with a 360°C.-10KOe annealing. Pinning layer for the stack is MnPt. MgO thicknessis controlled to less than 10 angstroms to give an RA of about 50 MRAMcircuits made of sub-100 nm MTJ cells were made using conventional deepUV lithography. For the 80 nm×160 nm MRAM MTJ, J_(c) at 10 ns pulses isabout 2.0×10⁶ A/cm². TMR at 400 mV bias is about 58% and the readmargin, TMR/R_(p) covariance, for 100 nm×200 nm cells is 7.5,corresponding to a R_(p) covariance of 7.8%.

J. Hayakawa et al. (“Current-driven magnetization switching inCoFeB/MgO/CoFeB magnetic tunnel junctions, Japn. J. Appl. Phys. V 44, p.1267 (2005)) has reported critical current densities of 7.8 and 8.8×10⁵and 2.5×10⁶ with 10 ns pulse width, for MTJ cells processed with 270,300 and 350° C. annealing. MgO barrier layer is about 8.5 angstromsthick, yielding a RA of about 10 ω-m². Intrinsic MR as a function of theannealing temperature for an MTJ stack formed ofCo₄₀Fe₄₀B₂₀/MgO/Co₄₀Fe₄₀B₂₀ with a 20 angstrom thick Co₄₀Fe₄₀B₂₀ freelayer are 49, 73 and 110% respectively. It is noted that the free layerin an MTJ processed at 270 and 300° annealing temperatures is amorphous.The pinning layer for the MTJ stack was IrMn.

Y. Huai et al. (“Spin transfer switching current reduction in magnetictunnel junction based dual spin filter structures” Appl. Phys. Lett. V87, p 222510 (2005)) have reported on spin-transfer magnetizationtransfer of a dual spin valve of the following configuration:

Ta/MnPt/CoFe/Ru/CoFeB/Al₂O₃/CoFeB/Spacer/CoFe/MnPt/Ta

It is noted that the free layer of the dual structure is made of a lowsaturation moment (approx. 1000 emu/cm³) amorphous CoFeB. The nominalMTJ size is 90 nm×140 nm. RA is about 20 ω-μm² and dR/R is about 20%.For a dual spin-filter (DSF) structure, the free layer experiences thespin transfer effect on both faces, so the critical current density hasbeen reduced to approx. 1.0×10⁶ A/cm².

C. Horng et al. (docket No. HMG06-042. “A novel MTJ to reducespin-transfer magnetization switching current”) is assigned to the sameassignee (Magic Technologies) as the present invention and fullyincorporated herein by reference. Horng et al. have produced an STT-MTJtest structure that includes a MTJ stack of the form:

Ta/NiCr/eMnPt/Co₇₅Fe₂₅/Ru7.5/Co₆₀Fe₂₀B₂₀-Co₇₅Fe₂₅/(NOX)MgO11/Co₆₀Fe₂₀B₂₀/Ta

Which is processed at 265° C.-2 hrs-10KOe annealing, so that theCo₆₀Fe₂₀B₂₀ remains amorphous. It is noted that the pinning layer isMnPt. RA of the MTJ is controlled to less than 10 ω-μm² and intrinsicdR/R is about 100%. For the 100 nm×150 nm size MTJ, patterned usingconventional photo-lithography of the 180 nm node technology, dR/R at0.1 V bias is about 70-80%. Due to the fact that no array wasconstructed, there was no determination of R_(p) covariance. However,the covariance for a conventional MRAM of the same basic MTJ structure,but 200 nm×325 nm was measured to about 3.5%. Extrapolation to the 100nm×150 nm size of the STT-MTJ predicts that the covariance would beabout 7%. This value would not be sufficient to provide a good readmargin.

The TMR sensor currently under production at Headway Technologies usesan MTJ element of the form:

Ta/Ru/IrMn/CoFe/Ru/CoFeB/CoFe/MgO/CoFe-NiFe/NiFeHf

In this configuration, the pinning layer is IrMn. TMR sensor size whenthe resistance measurements are made (i.e., unlapped) is 100 nm×500 nm.Patterning is done using conventional photo-lithography of the 180 nmnode technology. R_(p) covariance across the 6″ wafer for that sensorsize is about 3%. Scaling to a 100 nm×150 nm size, the covariance isprojected to be about 5%.

It should be noted that to obtain improvement in R_(p) covariance, thephoto-lithography using the 65-90 nm node technology, as is nowpracticed in semiconductor technology, would be viable.

C. Bilzer et al. (“Study of the dynamic magnetic properties of softCoFeB films”, J. Appl. Phys. V 100, 053903 (2006)) has measured themagnetization damping parameters for the ion-beam deposited Co₇₂Fe₁₈B₁₀film as a function of film thickness and crystalline state. AmorphousCo₇₂Fe₁₈B₁₀ showed low damping with a between 0.006 and 0.008, which wasthickness independent. Crystalline Co₈₀Fe₂₀ shows a damping factor thatis approximately a factor of 2 higher.

M. Oogane et al. (“Magnetic damping in ferromagnetic thin film”, Japn.J. Appl. Phys. V 45, p 3889 (2006)) have measured the Gilbert dampingfactor for the ternary Fe—Co—Ni and CoFeB films. As shown in FIG. 6, alow damping constant is measured for the Fe rich FeCo and the Fe—Nibinary alloys. For the CoFeB alloys, as shown in FIG. 7, the dampingconstant is 0.0038 and 0.010 respectively for amorphous Co₄₀Fe₄₀B₂₀ andCo₆₀Fe₂₀B₂₀.

The above prior art tends to imply that:

1. J_(c) is greater than 2×10⁶ A/cm² for the CoFeB/MgO/CoFeB MTJ with acrystalline CoFeB free layer.

2. J_(c) less than 1.0×10⁶ A/cm² is achievable for the CoFeB/MgO/CoFeBMTJ with an amorphous CoFeB free layer.

3. The R_(p) covariance for an STT-RAM MTJ made, using conventional 180nm node photo-lithography, with an MnPt (pinning)/CoFe(B)MgO/CoFeBstructure is greater than or equal to 7.5%, while a covariance that isless than or equal to 5% may be achievable for the MTJ with an IrMnpinning layer.

4. A low damping factor free layer is critical for reducing thespin-torque magnetization switching current.

An examination of the patented prior art shows an increasing number ofinventions utilizing the STT approach to MRAM switching. Although thisprior art describes many different MTJ stack configurations and layermaterials, none of them address the particular combination ofconclusions that we have drawn and that are listed above in 1. through4.

Shimazawa et al. (U.S. patent application Ser. No. 2007/0086120), Ashidaet al. (U.S. patent application Ser. No. 2007/0076469) and Huai et al.(U.S. patent application Ser. No. 2006/01022969) all teach an AFM layercomprising IrMn.

Nguyen et al. (U.S. Pat. No. 6,958,927) and Huai et al. (U.S. Pat. No.7,126,202) teach that a first AFM layer is preferably IrMn.

Huai et al. (U.S. Pat. No. 6,967,863) discloses that an AFM layer ispreferably IrMn or PtMn.

Huai et al. (U.S. Pat. No. 7,106,624) states that the AFM is preferablyPtMn but “nothing prevents” the use of IrMn instead.

Covington (U.S. Pat. No. 7,006,375) shows a pinned layer that can beeither IrMn or PtMn.

Pakala et al. (U.S. patent application Ser. No. 2006/0128038) disclosesthat seed layers may be used to provide a desired texture to the AFMlayer. For example, if IrMn is used as the AFM layer, then a TaN layershould be used.

The present invention will describe a spin transfer MRAM device in whicha new form of free layer, combined with an IrMn pinning layer willaddress the issues raised above in statements 1-4.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide an MTJ element foran STT-MRAM cell wherein the critical current for magnetizationswitching by spin-torque transfer (STT) is lowered.

A second object of this invention is to provide a MTJ element for anSTT-RAM configured MRAM device in which the intrinsic (unbiased) andoperational (biased) TMR ratio, dR/R, is significantly enhanced andwherein the intrinsic dR/R is between about 125% to 130%.

A third object of the present invention is to provide such a device inwhich the product RA, of MTJ resistance (R) and MTJ elementcross-sectional area (A) is less than a certain amount, preferably lessthan 10 ω-μm².

A fourth object of the present invention is to provide a MTJ elementpatterned using standard 180 nm photo-lithography and correspondingSTT-RAM configured MRAM array of such elements, wherein the statisticaldistribution of MTJ resistances has a low covariance, preferably lessthan 5%.

A fifth object of the present invention is to provide a MTJ element andcorresponding STT-RAM configured MRAM array of such elements, whereinthe read margin, (TMR/R_(p)covariance), is greater than 15, and for anelement of cross-sectional area 100 nm×150 nm is at least 20.

A sixth object of the present invention is to provide such an MTJelement wherein the structure of the pinning/pinned layer provides a lowangular dispersion of the pinned layer magnetic moment direction,preferably 1.6× less than that of conventional configurations.

These objects will be met by an MTJ element structure in which the freelayer damping constant (Gilbert damping constant a) is reduced to lessthan that associated with only a free layer of amorphous CoFeB, in whichthe tunnel barrier layer is deposited in a crystalline form by means ofbeing sputtered from an Mg target and then naturally oxidized, in whichhighly efficient and enhanced spin polarization is obtained bysurrounding the free layer by layers of crystalline Fe and in which theantiferromagnetic pinning layer is a layer of MnIr.

The preferred structure of the MTJ element is of the form:

Buffer layer/Pinninglayer//Co₇₅Fe₂₅23/Ru7.5/Co₆₀Fe₂₀B₂₀15-Co₇₅Fe₂₅(API)/Mg8-NOX-Mg4/Feelayer/Ta30/Ru

Where the general ordering of the layers is as in FIG. 2. In thepreferred embodiment, the free layer is of the form Fe 3/CoFeB 20/Fe 6,with the two interfacial Fe layers being very thin (respectively 3 and 6angstroms) and crystalline in structure for enhanced spin polarizationof the current, while the CoFeB layer sandwiched between them isamorphous in form to obtain a low Gilbert magnetic damping factor. Thetunneling barrier layer is a layer of MgO that is rendered crystallinein nature by being formed by a combination of a sputtering deposition ofMg and a natural oxidation process. The sputtering is onto an exposedsurface of AP1 that has been preconditioned by a plasma treatment torender it smooth/flat. This plasma pretreatment is, in turn, highlyadvantageous, given the fact that the pinning layer for the SyAF pinnedlayer is a layer of MnIr rather than the more usual MnPt. The MnIrpinning layer, in turn, allows an advantageous annealing process inwhich the anneal need only be carried out at 265° C. for between 1 and 2hours in a magnetic field of 10 K Oe. It is to be noted that other priorart processes form the MgO tunneling barrier layer by sputteringdirectly from an MgO target. This produces an amorphous layer which mustbe rendered crystalline by use of much higher annealing temperaturesthan are required in the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a prior-art MTJ MRAM devicelocated at the junction of word and bit lines.

FIG. 2 is a schematic cross-sectional view of a typical prior artspin-transfer device, including an MTJ element and current providingtransistor, that, in the present invention, will utilize a novelcombination of a pinning layer a tunneling barrier layer and a freelayer.

FIG. 3 a, FIG. 3 b and FIG. 3 c are schematic views of the fabricationof the layer structure of a preferred embodiment of the presentinvention

FIG. 4 is a table showing the magnetic performance properties of variousMTJ configurations.

FIG. 5 a and FIG. 5 b are graphs showing characteristic B-H plots for anMTJ element incorporating an MnPt pinning layer (FIG. 5 a) and an MnIrpinning layer (FIG. 5 b).

FIG. 6 is a graphical illustration of the distribution of Gilbertdamping factors for various ternary compositions of Fe—Co—Ni alloyfilms.

FIG. 7 is a graphical illustration of the variation of the Gilbertdamping factor for Co—Fe—B films of various compositions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiment of the present invention is an STT-RAM memorycell and an array of such cells, in which each cell incorporates an MTJelement that operates in the spin torque transfer (STT) mode. The MTJelement includes an IrMn pinning layer, an SyAP pinned layer, acrystalline barrier layer of naturally oxidized MgO and acrystalline-Fe-sandwiched CoFeB free layer having Gilbert dampingconstants that are lower than those associated with free layers of onlyamorphous CoFeB. The preferred MTJ stack configuration is:

BE/NiCr50/Ru20/MnIr70/Co₇₅Fe₂₅23(AP2)/Ru7.5/CoFeB₂₀- Co₇₅Fe₂₅6-7(AP1)/PT/ Mg8-NOX-Mg4/Fe3-Co₆₀ Fe₂₀ B₂₀ 20-Fe6(FL)/Ta

Referring to FIG. 3 a, FIGS. 3 b and 3 c, there will be schematicallyshown the process steps by which the stack configuration is formed. Wewill retain the essential elements of the numbering of FIG. 2.

Referring first to FIG. 3 a, there is shown the structure labeled BEabove, which denotes a bottom conducting line or electrode (300). Layer(1) is a layer of NiCr, which is a seed layer. Layer (2) is the pinninglayer, which is a layer of MnIr of 70 angstroms thickness. Layer (345)is the configuration Co₇₅Fe₂₅23(AP2)/Ru7.5/CoFeB₂₀-Co₇₅Fe₂₅6-7 (AP1) isa synthetic antiferromagnetic (SyAF) pinned layer, where AP1 (5) and AP2(3) denote the two ferromagnetic layers magnetized in antiparalleldirections and exchange coupled by the layer of Ru (4). As we shallexplain below, PT denotes a plasma treatment (55) that is applied to theupper surface of AP1 and is required to produce the desired objects ofthe invention.

Referring next to FIG. 3 b, there is shown the formation of an MgO (or,similarly, an AlOx) barrier layer (6) on the plasma treated surface(555) of AP1. The plasma treatment, which smoothes and renders flat the(555) surface, is required because layers formed on a MnIr/SyAFcombination such as in the present invention are typically rougher thanwhen formed on an MnPt/SyAF combination. Consequently, dR/R and H_(e)(H_(in)) pf the MnIr MTJ are not as good as that for the MnPt MTJ. Toyield high dR/R and low H_(in), the CoFe(B)/MgO/CoFeB interfaces must besmooth. This is particularly true for the MTJ configuration where thevery high dR/R is a result of coherent spin-dependent tunneling throughthe (001)CoFe(B)/(001)MgO/(001)CoFe(B) MTJ junctions.

To prepare a smooth/flat bottom electrode (the bottom electrode denotingthe entire portion of the element below the MgO tunneling barrier layer)prior to the MgO deposition, the plasma treatment (PT) process (55) hasbeen applied to the exposed surface of AP1. PT is a low power (20W) Arion-milling process that smoothes and flattens the AP1 surface. The ionenergy is sufficiently low so that it does not damage the surface.

Referring again to FIG. 3 b, the MgO barrier layer (6) is made by aprocess indicated above by Mg8-NOX-Mg4 and which is more fully describedby Horng et al cited above. Unlike prior art processes, in which atarget of MgO is sputtered and thereby produces an amorphous MgO layer,in the present process an Mg target is sputtered to produce acrystalline Mg layer and that already crystalline layer is thennaturally oxidized after deposition to directly produce a crystallineMgO layer. A subsequent second Mg sputtering process completes the layerformation as described by Horng. In its as-deposited state, thenaturally oxidized Mg already has a highly oriented (001) crystallineplane texture. In this respect, prior art rf-sputtered (extra-thin) MgOis deposited in an amorphous state and has to rely on subsequent hightemperature (>350° C.) annealing processing to obtain a highly oriented(001) structure. Such high temperature annealing is not required in thepresent invention.

Referring now to FIG. 3 c, there is shown the formation of the freelayer of the present invention. The free layer (7) of the configurationabove is a composite layer made of an atomic layer thickness(approximately 3 angstroms) crystalline Fe interface layer (71) andthicker, amorphous layer (72) of Co₆₀Fe₂₀B₂₀ (approximately 20angstroms) formed upon it. More preferably, as is shown in the figure,an additional crystalline layer (73) of Fe approximately 6 angstroms inthickness can be inserted between the Co₆₀Fe₂₀B₂₀ and the Ta 30 cappinglayer (8). These two layers of Fe provide advantageous enhanced electronpolarization properties at the interfaces between the MgO layer (6) andthe free layer (7), between the AP1 layer (5) and the MgO layer (6) andbetween the free layer (7) and the Ta layer (8). Finally, it is alsonoted from the evidence of FIG. 7 that a crystalline structure freelayer of the form Fe(rich)—Co/Fe(rich)—Ni, which is a bilayer of twobinary alloys that are rich in Fe, such as FeCo/FeNi can produce an evenlower damping factor than the Fe/CoFeB/Fe free layer with amorphousCoFeB shown in the present FIG. 3 c and, in addition, when subjected toa >300° C. (e.g. 350° C.) post annealing can yield a dR/R=200% and acorresponding read margin that is >25. Such a free layer will,therefore, produce another preferred embodiment of the presentinvention.

For comparison, we also made an STT-MTJ element in the configuration ofHorng et al., in which the pinning layer is MnPt and the free layer isonly amorphous Co₆₀Fe₂₀B₂₀, without the interfacial layers of Fe. Toobtain the amorphous Co₆₀Fe₂₀B₂₀ layer the MTJ stack is processed usingan (250-265° C.)-10KOe (1-2 hour) annealing treatment. Magneticproperties of the stack configurations are displayed in the table ofFIG. 4. It has already been noted here and will be discussed below, thatforming the Co₆₀Fe₂₀B₂₀ on the atomic layer of Fe produces a higherdegree of electron spin polarization than the amorphous layer alone.

Referring again to FIG. 4 there is seen a table setting forth themagnetic properties of 7 MTJ stack configurations having the generalform:

Buffer layer/Pinninglayer//Co₇₅Fe₂₅23/Ru7.5/Co₆₀Fe₂₀B₂₀15-Co₇₅Fe₂₅(AP1)/ Mg8-NOX-Mg4/Feelayer/Ta30/Ru

Row 1 shows the properties of the reference MTJ stack in which thepinning layer is MnPt and the free layer lacks the Fe crystalline layerand consists only of the amorphous CoFeB layer. Row 3 shows a similarstructure except that the pinning layer is MnIr and the free layerincludes the crystalline Fe layer. MR for row 1 is 103%, while for row 3it is 118%. This improvement is attributed to the (001) Fe crystallineinterface layer in the free layer structure.

Due to a rougher bottom electrode, H_(e) (H₁) for the MnIr-MTJ is 7.60Oe, a twofold increase over the value of 3.73 Oe for the MnPt MTJ of thereference structure. Row 2 and Row 4, which include a 120 sec. plasmatreatment (PT) of the AP1 surface, show that H_(e) of both the MnPt andMnIr TMJ have been reduced to 2.48 and 4.41 Oe respectively. Havingsmooth/flat interfaces at the AP1/MgO/free layer enhances the MR forboth the MnPt and MnIr structures. For 100 nm×150 nm MTJ elements, MR(at 0.1 V bias read operation) for the MnIr MTJ would be about 100%. Tomeet the read margin requirement of TMR/(R_(p) covariance) >20, thecovariance would have to be less than or equal to 5%.

To obtain an amorphous CoFeB layer for its low magnetization damping andlow M_(s), the deposited MTJ stack is annealed using a 265° C. (1-2 hrs)10K Oe process. Previously, in GMR sensor head fabrication, to obtain arobust MnPt/SyAF pinned layer, post deposition annealing to the MTJstack was done in a 280° C. (5 hrs)-10 K Oe process. Post depositionannealing for the MnIr/SyAF MTJ, however, is at 250°-265° C. (5 hrs)-10K Oe. It appears that the 265° C. (1-2 hrs)10K Oe process is moresuitable for the MnIr pinning layer structure than for the MnPt pinningstructure. In terms of TMR magnetic performance, the MnIr MTJ element ispreferred.

Referring now to FIGS. 5 a and 5 b, there is shown the B-Hcharacteristic plots for the MTJ element made (in 5 a) for the MnPtpinning layer and (5 b) for the MnIr pinning layer. The open loopportion of each curve (arrows) allows us to deduce the pinned layerdispersion and the strength of the pinning field for the MnIr MTJ andfor the MnPt MTJ.

Thus the ratio of H_(pin)(MnIr)/H_(pin)(MnPt) is approximately 1.6. Theratio of dispersion (MnPt)/dispersion (MnIr) is about 1.6. From this, wecan also deduce that the covariance ratio of [R_(p) cov. (MnPt)/R_(p)cov (MnIr)] is about 1.6. Using [R_(p) cov. (MnPt)]=7.5% given by Hornget al, we can deduce that [R_(p) cov. (MnIr)]=4.7%, which is in closeagreement to Headway Technology's measured MnIr stack covariance fortheir TMR sensor made with a MnIr pinning layer (described above).

Referring to FIG. 6, it is noted that the boron (B) doping to the CoFefree layer has enhanced the magnetization damping. Use of the Fecrystalline interface layers provides high spin polarization (higherthan from the amorphous CoFeB layer alone). As can be seen in FIG. 7,the damping constant of Fe is α=0.0019 and the damping constant ofCo₆₀Fe₂₀B₂₀ is α=0.01. The effective damping constant for the compositefree layer of Fe/Co₆₀Fe₂₀B₂₀/Fe (with the thicknesses of the two Felayers being approximately 3 and 6 angstroms respectively and thethickness of the Co₆₀Fe₂₀B₂₀ being approximately 20 ang.) is calculatedto be α=0.006, which will produce the required improvement of thecritical current for magnetic moment switching. It is further noted thatan even smaller damping constant can be obtained using a free layer ofFe/Co₄₀Fe₄₀B₂₀/Fe (same Fe thicknesses, but the sandwiched layer ofCoFeB being approximately 15 angstroms) since the damping constant ofCo₄₀Fe₄₀B₂₀ is α=0.0038, which is smaller than the α=0.01 of theCo₆₀Fe₂₀B₂₀.

As is finally understood by a person skilled in the art, the preferredembodiments of the present invention are illustrative of the presentinvention rather than limiting of the present invention. Revisions andmodifications may be made to methods, materials, structures anddimensions employed in forming and providing a CPP STT-MTJ MRAM cell,said cell using transfer of spin angular momentum, while still formingand providing such a cell and its method of formation in accord with thespirit and scope of the present invention as defined by the appendedclaims.

1. An STT-MTJ MRAM cell operating in a CPP configuration and utilizingthe transfer, by torque, of conduction electron spin angular momentum tochange a free layer magnetization direction, comprising: a substrate; anMTJ element formed on said substrate, said element comprising avertically stacked lamination of horizontal parallel layers including,therein, in the following order: an antiferromagnetic pinning layerformed of MnIr, an SyAP pinned layer processed by a plasma process tohave a smooth/flat interfacial surface, a tunneling barrier layer formedon said smooth/flat interfacial surface, said tunneling barrier layerhaving a crystalline structure and comprising a naturally oxidized,sputtered layer of Mg, a ferromagnetic free layer formed on saidtunneling barrier layer, said ferromagnetic free layer having a lowmagnetic damping factor and producing enhanced polarization ofconduction electrons, and a capping layer formed on said free layer, andwherein said ferromagnetic free layer is a composite tri-layercomprising at least one interfacial layer of crystalline ferromagneticmaterial and an amorphous ferromagnetic layer formed thereon andwhereby, a current of conduction electrons in the vertical direction canchange the direction of magnetization of said free layer relative to themagnetization direction of said pinned layer.
 2. The cell of claim 1wherein said composite tri-layer comprises a first crystalline layer ofFe of 3 angstroms thickness on which is formed an amorphousferromagnetic layer of Co₆₀Fe₂₀B₂₀ of approximately 20 angstromsthickness, on which is formed a second crystalline layer of Fe ofapproximately 6 angstroms thickness.
 3. The cell of claim 1 wherein saidcomposite tri-layer comprises a first crystalline layer of Fe of 3angstroms thickness on which is formed an amorphous ferromagnetic layerof Co₄₀Fe₄₀B₂₀ of approximately 15 angstroms thickness, on which isformed a second crystalline layer of Fe of approximately 6 angstromsthickness.
 4. The cell of claim 2 wherein said crystalline layers of Feare each formed with an (001) crystal plane parallel to its plane offormation.
 5. The cell of claim 3 wherein said crystalline layers of Feare each formed with an (001) crystal plane parallel to its plane offormation.